FAMILIAL AND EARLY ONSET COLORECTAL CANCER ABSTRACT Colorectal cancer (CRC) is a common and potentially lethal disease that usually occurs in older people, and is a sporadic process principally related to dietary and other environmental influences. However, about 4% of CRC can be attributed to specific genetic syndromes such as Lynch Syndrome, familial adenomatous polyposis, and a few other rare diseases. Another 20-30% of patients with CRC have a first degree relative with CRC, but it is not known how much of this familiality is due to shared genes rather than shared environmental factors. In any event, we have few methods of identifying these high-risk people. CRC is particularly suited to preventive strategies because of the availability of multiple effective screening modalities, but we cannot screen everyone frequently, and these resources would be more effective if they were used more intensively in those people at greatest risk for the disease, and more sparingly in those individuals at ordinary or lower risk. Also, about 5-10% of CRC occurs in people who are relatively young, <50 years old, and even ambitious screening recommendations are inadequate for these individuals. The goals of this project are to study individuals who are at increased risk for CRC on a familial basis, or for non-familial early-onset CRC, so that they might be offered appropriately intensive screening to mitigate their risk of dying of cancer. The focus of the application will be on alterations of the DNA mismatch repair (MMR) genes. We have used collaborations to accumulate a large number of routine and unique CRC specimens to look for a number of previously unexplored possibilities. Methylation-induced silencing of the MLH1 gene occurs because of a CpG island in its promoter, and accounts for about 12% of CRCs. We have developed a unique model to study the regulation of methylation of the MLH1 gene in vitro and in vivo. We will test the hypothesis that a unique demethylating agent discovered in our laboratory can reverse this process in vitro and in vivo. We will also look for evidence of methylation-induced silencing of the MSH2 and MSH6 genes, which also have CpG islands in their promoters, and should be susceptible to the same perturbation. We will look for mechanisms responsible for early-onset CRC in patients who do not appear to have a family history of this by testing a new panel of microsatellite markers for microsatellite instability (MSI), by looking for the promoter methylator phenotype in these tumors, and by using novel methods to look for low-level MSI. Finally, we will test the hypothesis that low-level MSI is caused by the down-regulation of the MSH3 gene, and that this process facilitates the generation of highly metastatic clones within a growing tumor mass. The broad aim of this application is to develop additional tools that increase our ability to more precisely categorize CRCs, to develop more accurate interpretations of the mutational signatures in CRCs, which will permit us to deliver more highly personalized treatment to CRC patients, particularly those who are young or have positive family histories of this disease.